The paper examines the incentives of Governments to introduce green taxes instead of emission licenses. We take into account the fact that licenses are durable goods, and distinguish Governements that may have fiscal goals as well as ecological ones. Taxes solve the credibility problem a monopoly supplier of licenses would face, and lead to a suboptimal low emission level in each period. From welfare point of view, our model provides a new argument in favor of licenses that is different from the traditional arguments (like informational asymmetries or incentives to innovate).